Medium density fiberboard (MDF) is a wood or agricultural fiber based composite material that draws on the usage of fibers, rather than particles or veneers, to produce board or sheet products. Although it is typically made as a panel or sheet, its use in decorative and structural products is increasing. It is also increasingly replacing particleboard in uses such as furniture manufacture, cabinet making, craft work, and flooring. Its advantages include high strength, ease of machining, good weathering properties, and the ability to be made from a wide variety of fibrous products including recycled materials.
FIG. 1 is a block diagram of an exemplary composite panel manufacturing process 100. In step 105 of FIG. 1, raw material is furnished. The primary constituent of MDF is typically a softwood that has been broken down into fibers, that is, the very cells (tracheids, vessels, fibers and fiber-tracheids), which are far smaller entities than those used in particleboard. The furnish for MDF normally consists of wood chips, or other agricultural residues, that may be delivered from offsite locations such as sawmill, plywood plants, furniture manufacturing facilities, chip mills, whole tree chipping operations, or farms. Alternatively, the wood chips may be prepared onsite. For example, whole logs are debarked, cut to manageable lengths, and sent to chippers which cut the logs into chips. The fiber materials are then softened in step 110, typically, in a steam-pressurized digester and processed in a refiner chamber (e.g., refining 115 of FIG. 1). In the refiner chamber, single or double revolving disks are used to mechanically pulp the softened materials into fibers suitable for making the board.
After defibration, the fibers are conducted at a high velocity through a blowline. In the blowline, wax is added to the fibers to improve the moisture resistance of the finished board. Additionally, an adhesive resin is added to the wet fibers. This is shown in step 120 of FIG. 1. There are a variety of adhesive resins used in MDF fabrication including urea formaldehyde, phenol formaldehyde, phenolic resins, melamine resins, and isocyanates, or hybrid combinations of these resins. The agitation of fibers in the blowline helps disperse the adhesive resin more consistently throughout the mass of fibers. The resinated fibers are then discharged into a dryer in step 125 to remove excess moisture from them. A cyclonic collector (e.g., fiber recovery 130 of FIG. 1) is often used to collect the resinated fibers after they have been dried.
In step 135, forming of the fiber occurs in which the fiber is typically deposited to a uniform thickness upon a continuous mat or screen. The mat may be pre-pressed before being fed to a hot press (e.g., hot press 140 of FIG. 1) that applies heat and pressure to activate the resin and bond the fibers into a compressed solid panel. The mat may be pressed in a continuous hot press, or the pre-compressed mat may be cut into individual mats before being sent to a multi-daylight hot press. After pressing, the boards are dried and cooled in step 145, before being trimmed, sanded, and sawed to final dimensions in step 150. The boards are typically stored for a few days to allow complete curing of the adhesive resins before being shipped in step 155.
Fire retardant composite panels are used to resist fire and thereby increase the time available for people trapped in burning buildings to escape. Most countries have developed standard test methods for assessing fire retardancy of building materials and building codes that define when such materials must be used in construction and what their properties should be. There are a variety of chemicals that are used to induce fire resistance in composite panels, however, boron and ammonium polyphosphate salts are the most commonly used fire retardant chemicals. For commercial purposes, fire retardant chemicals should be inexpensive, proven to be effective, and readily available to the manufacturer. The most common of these in commercial applications are ammonium polyphosphates and various boron compounds such as boric acid and borax.
Boron based fire retardant chemical treatments are advantageous in that they are less expensive, and provide a greater degree of resistance to smoke development than is realized with polyphosphates in some international standards tests. Additionally, boron based fire retardants allow the use of urea formaldehyde as an adhesive which is less expensive than other commonly used adhesive resins. However, boron based fire retardant chemicals are only partially soluble in water and cannot be made into solutions strong enough to practically impart a sufficient degree of fire retardance to comply with many international standards. Thus, they are typically incorporated as dry powders to the fiber based furnish. For example, some of the earliest fire retardant composite panels manufactured commercially were particle boards that utilized dry boron salts that were mixed with the wood particles in controlled proportions. Recently, some manufacturers of MDF have mimicked the production of boron based fire retardant particle board processes and are adding dry boron salts to their fiber.
Unfortunately, the granular nature and poor dispersion of the dry chemicals requires that relatively high concentrations be used in the panel to gain a desired level of fire resistant performance. The granular boron salts frequently clump into localized regions of high chemical density that can react with overlying veneers and finish products that are applied to the panels and/or weaken the panel by reacting with the adhesive resins.
Ammonium polyphosphate salts tend to be soluble in water and can be readily incorporated into composite panel fabrication processes. They are advantageous over dry chemicals in that the solution provides a more even distribution of the fire retardant chemicals over the particles with fewer regions of concentrated chemicals. As a result, manufacturers of fire retardant MDF panels typically use ammonium polyphosphate. However, the polyphosphates are more expensive chemicals than the dry boron salts and often have adverse effects on the strength and water resistance properties of the panel. As a result, manufacturers have to either accept lower physical properties for their products, or limit the application of fire retardant chemicals to higher density products. Alternatively, some manufacturers use specialized, and more expensive, adhesives to attain a level of performance comparable to non fire retardant panels.
Another manufacturing process for MDF incorporates boric acid and borax fire retardant chemicals by suspending the particles of boron salts in the adhesives used to bind the fibers. This enables utilizing the less expensive urea-formaldehyde adhesives without affecting the properties of the panels. However, the process has several disadvantages. For example, the ratio of fire retardant salts to adhesive is difficult to vary. Thus, if more adhesive is needed to achieve a desired property of the panel, more fire retardant is also incorporated. Conversely, if less adhesive is needed to achieve the desired physical properties of the panel, it is still limited by the need to have minimum fire retardant chemical loadings in the fiber.
A second disadvantage to this process is a limit to the amount of dry chemicals which can be added to the resin slurry before it becomes impractical to pump the slurry. As a result, the amount of dry chemicals that can be added to the adhesives is limited. This is disadvantageous because in some instances, international standards for fire resistance cannot be met, or are on the lower limit of acceptance. As an alternative, additional water can be added to the slurry mixture to facilitate pumping the slurry. However, the addition of water affects the adhesive properties of the resin and sometimes results in reduced productivity as additional time and energy are expended to remove the additional water from the fiber in the drying stage. Another disadvantage is that the pH of the slurry causes the acid cured adhesive to begin to pre-cure. Thus, the resin based slurries have a limited shelf life, typically one day, before they should be discarded.
Another disadvantage in conventional fire resistant composite panel fabrication processes is that the dry fire retardant chemicals cannot be economically acquired in bulk in a finely divided form. It is recognized that the dry chemicals are best incorporated into the composite panel in a finely divided form. However, this necessitates additional processing to create a usable fine powder that adds to the expense of fabricating fire retardant MDF products. Additionally, processing of the boron salt solids into usable fine powders is becoming increasingly difficult in order to comply with stricter environmental and safety regulations that regulate dust levels and aerial contamination.
Because of the greater expense of the fire retardant chemicals, the special adhesives that are necessary, and the effects of the fire retardant chemicals on the properties of the panels, fire resistant MDF tends to be more expensive than fire retardant particle board. Thus, in spite of its advantages in some “high-end” applications, fire resistant MDF has realized only limited commercial acceptance due to the additional costs associated with its manufacture.